The invention concerns a device for welding the ends of tubular containers made from plastic, in particular tubes.
A large number of pourable or pasty media are packed in tubes, e.g. cosmetics, tooth paste, shoe polish, creams and the like. The tubes can be made from a suitable malleable flat metallic material. However, plastic material has been utilized for some time, primarily for economical reasons.
The tubular container blank is initially filled with the material to be contained prior to sealing the end. The removable side is sealed with a suitable cap, such as a screw cap or the like. The fill end is sealed following filling. When malleable flat material is used, a fold line has been conventionally effected on the fill end. Optionally, a linear pressing is carried out in the end region to thereby guarantee adequate sealing. This type of method cannot be utilized for plastic materials. Rather, the adjoined end portions must be welded or sealed together. DE 37 44 402 C2 discloses a method for sealing tubular containers made from plastic, the end sections of which are initially softened or melted through the introduction of heat or the like before producing a sealing seam with the assistance of a pressing device.
The softening is thereby effected by disposing a ring about the filling end having a plurality of nozzle openings to blow hot air onto the outer wall of the fill end of the tube. Since complete welding is normally required, substantial amounts of heat must be introduced externally, i.e. requiring up to approximately 18 kW of electrical power. This method is currently utilized in the packaging industry and is referred to as the xe2x80x9chot airxe2x80x9d method.
In analogy to the hot air method, a softening or melting of the end which is to be welded has also conventionally been effected with the assistance of ultrasound or microwave radiation.
All these methods require the melting and the subsequent pressing together to be effected in sequential production steps, since the device for melting the material cannot assume the same spatial position as the pressing device. In addition, it is normally necessary to cool down the initially heated tube following the pressing procedure, since an undesirable malleable deformation could otherwise occur and since the product could also be damaged by the heat. As a result of this additional cooling step, this method is extremely wasteful.
DE 22 61 388 discloses a method for welding an end piece made from plastic onto a tubular plastic body, wherein the end piece, having a tubular section, is inserted into the tubular body. A laser is used to irradiate the outer side of the tubular body, whereby the tube and the end piece rotate. A pressing roller presses the warmed regions together and effects a type of welding.
It is the underlying purpose of the invention to create a method and a device for welding the ends of tubular containers, in particular tubes, which can be used for all conventional types of tube plastic and which requires little energy consumption with high production speed and modest technical effort.
This purpose is achieved with the features of the independent claim.
With the device in accordance with the invention, a preferentially segmented holder is provided for a plurality of laser diode units disposed approximately in a circle so that their beams are incident on the outer or inner wall of the container, either directly or via a deflection optics.
The wavelength of diode lasers is in the near infrared (approximate 650 nm to 1400 nm) and it is irrelevant for the effectiveness of the device whether or not the inner or the outer wall of the container is irradiated, since nearly all plastics which could be used are partially absorbing and the radiation can thereby be evenly active throughout the entire wall thickness (less than 1 mm). In order to simplify the mechanical construction, the outer wall should be irradiated. The melting of the inner wall, required for the pressing together procedure, is thereby effected through irradiation of the outer wall. The method in accordance with the invention provides for a homogenous heating of the melting zone. In order to improve the effectiveness, it is also possible to dispose a reflector inside the container, on that side facing away from the laser diodes, to reflect the radiation back into the material.
Beams emanating from a laser diode or from a laser diode array are strongly divergent, generally in a range between approximately 6 to 10 degrees in the plane of longitudinal extension of the laser diode (slow axis) and approximately 35 to 40 degrees in the plane of transverse extension of the laser diode (fast axis) perpendicular to the longitudinal extension. This situation can be partially taken advantage of in the device in accordance with the invention. In order to effect a continuous joining of the laser projections about the periphery of the container, one can assume that the fast perpendicular axis of the laser radiation (approximately 10 degrees) determines the radial separation of the laser diode units, disposed in a circle, from the container. In order to effect a continuous radiation in the peripheral direction, one should subtract a value of approximately 0.2 degrees from the value of the parallel radiation angle of the fast axis.
The laser diode radiation should advantageously be focussed to achieve sufficient power density (according to current experience, the power density should be  greater than 20 W/cm2). In order to achieve this power density, e.g. the slow axis of the laser diode unit is strongly focused. A focus of approximately 0.5 mm can be achieved without any particular difficulty. On the other hand, the laser radiation irradiated in the peripheral direction of the container is e.g. only weakly focussed for adjusting the radiation pattern. An optimal focusing can thereby be effected as follows. The slow axis is focused to about 0.5 mm. The fast axis is not focused and maintains a radiation angle of approximately 45 degrees. In this fashion, one laser unit irradiates the periphery of the tubular body through a peripheral area of 0.5 mmxc3x9745 degrees. With a tube diameter of 25 mm, 40 degrees corresponds to 9.8 mm. This results in a irradiation area of 4.9 mm2. With a laser output power of 20 W, this leads to a power density of approximately 40 W/cm2.
An aspherical lens can be utilized to adjust the focus of the laser beam to the curvature of the cylindrical surface. In order to avoid this situation, the number of laser diode units can also be increased until the focal depth is sufficient to compensate for the curvature. The fast radiation axis is thereby focused in such a fashion that a radiation angle of 10 degrees is achieved. The 10 degree angle leads to a reduced curvature value on the surface of the tubular container which, under certain circumstances, must no longer be compensated for optically.
The mechanical positioning of the laser diodes likewise depends on the focus. The relative positions of the laser diode units necessarily change with increasing container circumference. The power density on the container wall thereby decreases proportionally. In this case as well, the laser diode units can be disposed at mutual separations corresponding to only a fraction of the fast axis radiation angle to compensate for this reduction in power density. As already mentioned above, this reduces problems related to the curvature of the container surface.
In a simple configuration, the laser diode units can be disposed in correspondence with their normal radiation characteristics (approximately 45 degree perpendicular radiation angle). This leads to eight laser diode units disposed about the axis of the container. In order to increase the power density of the above mentioned configuration by up to 200%, the axial separation between the laser diode units must be reduced to an angle of approximately 22.5 degrees.
In order to adapt to the diameter of the tubular container or to change the energy density at the point of incidence, an embodiment of the invention provides that the laser diode units can be displaced in the direction of their longitudinal axis. A suitable displacement device is correspondingly provided and adapted in such a fashion as to effect displacement of the laser diode units.
A circular or ring-shaped structure can be utilized for bearing and displacing the laser diode units which consists essentially of individual segments, each supporting a laser diode. The number of segments thereby corresponds to the number of laser diode units. Each segment can be individually borne for displacement along the longitudinal axis. All segments can be disposed together on a ring-shaped support.
An additional embodiment of the invention provides that the height of the segment support is adjustable for all segments. This permits change of the position of incidence on the outer wall of the container.
Instead of the above mentioned laser diode units, fiber coupled diode lasers or fiber lasers can be analogously utilized. The output optics must be adjusted in dependence on the type of laser.
The invention is described more closely with regard to embodiments shown in the drawing.